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    SubjectRe: [RFC][PATCH 12/12 v3] x86/nmi: Perform a safe NMI stack trace on all CPUs
    On Tue 2014-11-04 10:52:49, Steven Rostedt wrote:
    > From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
    >
    > When trigger_all_cpu_backtrace() is called on x86, it will trigger an
    > NMI on each CPU and call show_regs(). But this can lead to a hard lock
    > up if the NMI comes in on another printk().
    >
    > In order to avoid this, when the NMI triggers, it switches the printk
    > routine for that CPU to call a NMI safe printk function that records the
    > printk in a per_cpu seq_buf descriptor. After all NMIs have finished
    > recording its data, the trace_seqs are printed in a safe context.
    >
    > Link: http://lkml.kernel.org/p/20140619213952.360076309@goodmis.org
    >
    > Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    > ---
    > arch/x86/kernel/apic/hw_nmi.c | 90 ++++++++++++++++++++++++++++++++++++++++---
    > 1 file changed, 85 insertions(+), 5 deletions(-)
    >
    > diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
    > index 6a1e71bde323..6e7bb0bc6fcd 100644
    > --- a/arch/x86/kernel/apic/hw_nmi.c
    > +++ b/arch/x86/kernel/apic/hw_nmi.c
    > @@ -18,6 +18,7 @@
    > #include <linux/nmi.h>
    > #include <linux/module.h>
    > #include <linux/delay.h>
    > +#include <linux/seq_buf.h>
    >
    > #ifdef CONFIG_HARDLOCKUP_DETECTOR
    > u64 hw_nmi_get_sample_period(int watchdog_thresh)
    > @@ -29,14 +30,35 @@ u64 hw_nmi_get_sample_period(int watchdog_thresh)
    > #ifdef arch_trigger_all_cpu_backtrace
    > /* For reliability, we're prepared to waste bits here. */
    > static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
    > +static cpumask_var_t printtrace_mask;
    > +
    > +#define NMI_BUF_SIZE 4096
    > +
    > +struct nmi_seq_buf {
    > + unsigned char buffer[NMI_BUF_SIZE];
    > + struct seq_buf seq;
    > +};
    > +
    > +/* Safe printing in NMI context */
    > +static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
    >
    > /* "in progress" flag of arch_trigger_all_cpu_backtrace */
    > static unsigned long backtrace_flag;
    >
    > +static void print_seq_line(struct nmi_seq_buf *s, int last, int pos)

    I would rename the arguments:

    "last -> first"
    "pos -> last"

    or maybe better would be to pass first positon and len.

    > +{
    > + const char *buf = s->buffer + last;
    > +
    > + printk("%.*s", (pos - last) + 1, buf);
    > +}


    > +{
    > + const char *buf = s->buffer + last;
    > +
    > + printk("%.*s", (pos - last) + 1, buf);
    > +}
    > +
    > void arch_trigger_all_cpu_backtrace(bool include_self)
    > {
    > + struct nmi_seq_buf *s;
    > + int len;
    > + int cpu;
    > int i;
    > - int cpu = get_cpu();
    > + int this_cpu = get_cpu();
    >
    > if (test_and_set_bit(0, &backtrace_flag)) {
    > /*
    > @@ -49,7 +71,17 @@ void arch_trigger_all_cpu_backtrace(bool include_self)
    >
    > cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
    > if (!include_self)
    > - cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
    > + cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_mask));
    > +
    > + cpumask_copy(printtrace_mask, to_cpumask(backtrace_mask));
    > + /*
    > + * Set up per_cpu seq_buf buffers that the NMIs running on the other
    > + * CPUs will write to.
    > + */
    > + for_each_cpu(cpu, to_cpumask(backtrace_mask)) {
    > + s = &per_cpu(nmi_print_seq, cpu);
    > + seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
    > + }
    >
    > if (!cpumask_empty(to_cpumask(backtrace_mask))) {
    > pr_info("sending NMI to %s CPUs:\n",
    > @@ -65,11 +97,57 @@ void arch_trigger_all_cpu_backtrace(bool include_self)
    > touch_softlockup_watchdog();
    > }
    >
    > + /*
    > + * Now that all the NMIs have triggered, we can dump out their
    > + * back traces safely to the console.
    > + */
    > + for_each_cpu(cpu, printtrace_mask) {
    > + int last_i = 0;
    > +
    > + s = &per_cpu(nmi_print_seq, cpu);
    > + len = s->seq.len;

    If there is an seq_buf overflow, the len might be size + 1, so we need to do:

    len = min(s->seq.len, s->size);

    Well, we should create a function for this in seq_buf.h.
    Alternatively, we might reconsider the overflow state,
    use len == size and extra "overflow" flag in the seq_buf struct.


    > + if (!len)
    > + continue;
    > +
    > + /* Print line by line. */
    > + for (i = 0; i < len; i++) {
    > + if (s->buffer[i] == '\n') {
    > + print_seq_line(s, last_i, i);
    > + last_i = i + 1;
    > + }
    > + }
    >
    > + if (last_i < i - 1) {

    IMHO, this should be:

    if (last_i < i)

    because last_i = i + 1. Otherwise, we would ignore state when there is
    one character after a new line. For example, imagine the following:

    buffer = "a\nb";
    len = 3;

    it will end with:

    last_i = 2;
    i = 3;

    and we still need to print the "b".

    > + print_seq_line(s, last_i, i);

    If I get it correctly, (i == len) here and "printk_seq_line"
    print_seq_line() prints the characters including "pos" value.
    So, we should call:

    print_seq_line(s, last_i, i - 1)

    > + pr_cont("\n");
    > + }
    > + }
    > +

    I hope that I have got it correctly. It is getting late here and I
    feel tired to see the off-by-one problems clearly ;-)

    Best Regards,
    Petr


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